Previously, we developed an experimental "rescue" system for human parainfluenza virus type 3 (PIV3) based on a cDNA-encoded minigenome, PIV3-CAT. This is a truncated version of PIV3 genome RNA in which all of the viral genes have been removed and replaced by the bacterial chloramphenicol acetyl transferase (CAT) gene. When synthesized in vitro and transfected into cells which have been infected with standard PIV3 helper virus, PIV3-CAT was rendered biologically active ("rescued") as evidenced by the intracellular expression of CAT and by the production of a component (presumably infectious particles) that was released into the medium and could be passed productively onto fresh cells in the presence of helper virus. Together with a similar system developed by us for human respiratory syncytial virus (RSV) (accompanying report) and by others for Sendai virus, this represents the first experimental method by which one can synthesize, and thereby manipulate, biologically-active analogs of the genome of a nonsegmented negative strand virus. It will now be possible to directly identify and characterize cis-acting signals in genome and, as described here, considerable progress has been made in this area. The success in rescuing PIV3-CAT supports the idea that it will be possible to adapt the system such that a complete synthetic genome can be used to produce nondefective virus. This would make possible the creation of viruses that contain defined mutations for vaccine and experimental purposes.